Earthquakes: Tremors from Below

Earthquakes: Tremors from Below

Earthquakes are happening all the time. Most are so small that people don’t even feel them. But some can be very powerful. If they occur near towns and cities, they can make buildings and bridges sway or even collapse.

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Image courtesy of AFP/Getty

Image courtesy of AFP/Getty

Earthquakes can also occur along the seafloor and cause it to move. And if the quake is powerful enough, it can trigger waves that travel across the ocean. These waves grow taller as they approach land, becoming a tsunami. Tsunamis can have devastating effects on people.

What causes earthquakes? And how do scientists study them?

Let’s find out.

Tsunami Sweeps South Pacific

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On the morning of September 29, 2009, a strong undersea earthquake occurred near the Samoan island chain in the South Pacific. It happened as children were getting ready for school and adults were dressing for work.

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“The ground shook from side to side and then swayedas if one was sitting on a boat on the water.”

Tsunami Survivor
Recalling the earthquake before the wave

Tsunami evacuation sign.

When the shaking stopped, people noticed the sea was rough and bubbly. Then it drained rapidly away from the shore. Within minutes, a tsunami swept over the islands. In some places, its height reached around 40 feet (12 meters). The earthquake had triggered the tsunami.

The enormous wave took 192 lives in Samoa and surrounding islands.

Coming Together

The people of Samoa helped each other rebuild homes and lives. They also held ceremonies to strengthen the connections within their community.

One ritual is for the leaders to pass a cup of the ceremonial drink called áva. The daughter of a high chief prepares áva by mixing pepper roots in a wooden bowl. During the ceremony, áva is passed around the group in a communal cup. Before drinking, each person pours a few drops on the ground, returning the áva to the earth.

Explore a Samoan ceremony.

Samoan ceremonies have many rituals. One ritual is for the leaders to pass a cup of the ceremonial drink called áva.

What causes the Earth to shake?

Tsunamis like the one in Samoa are triggered by large undersea earthquakes. To understand how tsunamis form, let’s first take a look at what causes earthquakes. Most earthquakes happen at the edges of tectonic plates, the big pieces of Earth’s crust that make up its outer shell. These plates fit together like a puzzle, but they are always moving very slowly, about one or two inches per year. Where the edges of the plates meet, three things can happen. Plates can collide, separate, or grind against each other. The movement between two plates can trigger earthquakes.

Drag the slider to reveal tectonic plate boundaries.

When Plates CollideWhen plates collide, sometimes one plate sinks, or subducts, below another one. This can trigger an earthquake. When earthquakes cause the ocean floor to move, the water moves, too. Columns of water travel across the ocean and grow taller as they approach land, becoming a tsunami.

Watch what happens when one plate sinks below another.

When Plates GrindWhen the edge of one plate grinds against another, it sometimes gets stuck and pressure builds up. When it finally breaks loose, rocks break, releasing energy in the form of a seismic wave, or earthquake.

Watch what happens when one plate grinds against another.

See how the dark layers on either side of this rock sample do not match up? This is a fault. Scientists study faults to learn how Earth’s crust moves over time.

How do scientists study earthquakes?

Around five million people are affected by earthquakes every year — so it’s not surprising that people want to know when the next big one will hit. But earthquakes are unpredictable. That’s because there’s no way to tell exactly when the rocks will snap from stress that builds up as the plates move. So what can scientists do? They calculate the probability that an earthquake will occur at a particular location.

Explore some of the tools that scientists use to measure, record, and study Earth’s movements.

Seismometers measure the seismic activity, or shaking, in Earth’s crust. Today, networks of hundreds of seismometers are at work around the world. They measure more than 100,000 earthquakes every year!

Photo by Karl Hagel/USGS

Seismometers measure the seismic activity, or shaking, in Earth’s crust. Today, networks of hundreds of seismometers are at work around the world. They measure more than 100,000 earthquakes every year!

Photo by Karl Hagel/USGS

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A seismogram is a recording of the earthquake’s intensity. The greater the peaks on the graph, the greater the shaking — and the greater the shaking, the greater the earthquake’s magnitude.

When an undersea earthquake occurs, scientists use computer models to predict whether a tsunami will occur. The model can forecast the wave’s speed, direction, and height as it approaches land. Local authorities can then warn communities that might be in danger.

Check out the path of a tsunami in this computer model.

“If we can understand earthquakes
and figure out which areas are subject to seismic hazard,we hope that we can save lives and property.”